A. Nicotinic Acid as an Antilipolytic Agent
Atherosclerosis and stroke are the numbers one and number three leading causes of death of both men and women in the United States. [See, e.g., Nature Medicine, Special Focus on Atherosclerosis, (2002) 8:1209-1262; the disclosure of which is hereby incorporated by reference in its entirely.] Type 2 diabetes is a public health problem that is serious, widespread and increasing [Brownlee M, Nature (2001) 414:813-20 and references therein; Zimmet P et al., Nature (2001) 414:782-7 and references therein; Saltiel A R et al., Nature (2001) 414:799-806 and references therein; the disclosure of each of which is hereby incorporated by reference in its entirety]. Elevated levels of low density lipoprotein (LDL) cholesterol or low levels of high density lipoprotein (HDL) cholesterol are, independently, risk factors for atherosclerosis and associated cardiovascular pathologies. In addition, high levels of plasma free fatty acids are associated with insulin resistance and type 2 diabetes. One strategy for decreasing LDL-cholesterol, increasing HDL-cholesterol, and decreasing plasma free fatty acids is to inhibit lipolysis in adipose tissue. This approach involves regulation of hormone sensitive lipase, which is the rate-limiting enzyme in lipolysis. Lipolytic agents increase cellular levels of cAMP, which leads to activation of hormone sensitive lipase within adipocytes. Agents that lower intracellular cAMP levels, by contrast, would be antilipolytic.
It is also worth noting in passing that an increase in cellular levels of cAMP down-regulates the secretion of adiponectin from adipocytes [Delporte, M L et al. Biochem J (2002) 367:677-85; the disclosure of which is incorporated by reference in its entirety]. Reduced levels of plasma adiponectin have been associated with metabolic-related disorders, including atherosclerosis, coronary heart disease, stroke, insulin resistance and type 2 diabetes [Matsuda, M et al. J Biol Chem (2002) 277:37487-91 and reviewed therein; the disclosure of which is hereby incorporated by reference in its entirety]. [Also see: Yamauchi T et al., Nat Med (2002) 8:1288-95; and Tomas E et al., Proc Natl Acad Sci USA (2002) Nov 27; the disclosure of each of which is hereby incorporated by reference in its entirety.] Globular adiponectin protected ob/ob mice from diabetes and apoE deficient mice from atherosclerosis [Yamauchi, T et al. J Biol Chem (2002) November; the disclosure of which is hereby incorporated by reference in its entirety]. [Also see Okamoto, Y et al. Circulation (2002) 26:2767-70; the disclosure of which is hereby incorporated by reference in its entirety.] There is evidence that the regulation of human serum adiponectin levels through modulation of adipocyte intracellular cAMP level is independent of adipocyte lipolysis [Staiger H et al., Horm Metab Res (2002) 34:601-3; the disclosure of which is hereby incorporated by reference in its entirety].
Nicotinic acid (niacin, pyridine-3-carboxylic acid) is a water-soluble vitamin required by the human body for health, growth and reproduction; a part of the Vitamin B complex. Nicotinic acid is also one of the oldest used drugs for the treatment of dyslipidemia. It is a valuable drug in that it favorably affects virtually all of the lipid parameters listed above [Goodman and Gilman's Pharmacological Basis of Therapeutics, editors Harmon J G and Limbird L E, Chapter 36, Mahley R W and Bersot T P (2001) pages 971-1002]. The benefits of nicotinic acid in the treatment or prevention of atherosclerotic cardiovascular disease have been documented in six major clinical trials [Guyton J R (1998) Am J Cardiol 82:18U-23U]. Structure and synthesis of analogs or derivatives of nicotinic acid are discussed throughout the Merck Index, An Encyclopedia of Chemicals, Drugs, and Biologicals, Tenth Edition (1983), which is incorporated herein by reference in its entirety.
Nicotinic acid and currently existing analogs thereof inhibit the production and release of free fatty acids from adipose tissue, likely via an inhibition of adenylyl cyclase, a decrease in intracellular cAMP levels, and a concomitant decrease in hormone sensitive lipase activity. Agonists that down-regulate hormone sensitive lipase activity leading to a decrease in plasma free fatty acid levels are likely to have therapeutic value. The consequence of decreasing plasma free fatty acids is two-fold. First, it will ultimately lower LDL-cholesterol and raise HDL-cholesterol levels, independent risk factors, thereby reducing the risk of mortality due to cardiovascular incidence subsequent to atheroma formation. Second, it will provide an increase in insulin sensitivity in individuals with insulin resistance or type 2 diabetes. Unfortunately, the use of nicotinic acid as a therapeutic is partially limited by a number of associated, adverse side-effects. These include flushing, free fatty acid rebound, and liver toxicity.
Agonists of antilipolytic GPCRs having limited tissue distribution beyond adipose may be especially valuable in view of the diminished opportunity for potentially undesirable side-effects.
The rational development of novel, nicotinic acid receptor agonists that have fewer side-effects is an area of active investigation, but to date it has been hindered by the inability to molecularly identify the nicotinic acid receptor. Recent work suggests that nicotinic acid may act through a specific GPCR [Lorenzen A, et al. (2001) Molecular Pharmacology 59:349-357 and reviewed therein; the disclosure of which is hereby incorporated by reference in its entirety]. Furthermore, it is important to consider that other receptors of the same class may exist on the surface of adipocytes and similarly decrease hormone sensitive lipase activity through a reduction in the level of intracellular cAMP but without the elicitation of adverse effects such as flushing, thereby representing promising novel therapeutic targets.
B. G Protein-Coupled Receptors
Although a number of receptor classes exist in humans, by far the most abundant and therapeutically relevant is represented by the G protein-coupled receptor (GPCR) class. It is estimated that there are some 30,000-40,000 genes within the human genome, and of these, approximately 2% are estimated to code for GPCRs. Receptors, including GPCRs, for which the endogenous ligand has been identified, are referred to as “known” receptors, while receptors for which the endogenous ligand has not been identified are referred to as “orphan” receptors.
GPCRs represent an important area for the development of pharmaceutical products: from approximately 20 of the 100 known GPCRs, approximately 60% of all prescription pharmaceuticals have been developed. For example, in 1999, of the top 100 brand name prescription drugs, the following drugs interact with GPCRs (the primary diseases and/or disorders treated related to the drug is indicated in parentheses):
Claritin ® (allergies)Prozac ® (depression)Vasotec ® (hypertension)Paxil ® (depression)Zoloft ® (depression)Zyprexa ® (psychotic disorder)Cozaar ® (hypertension)Imitrex ® (migraine)Zantac ® (reflux)Propulsid ® (reflux disease)Risperdal ® (schizophrenia)Serevent ® (asthma)Pepcid ® (reflux)Gaster ® (ulcers)Atrovent ® (bronchospasm)Effexor ® (depression)Depakote ® (epilepsy)Cardura ® (prostatic ypertrophy)Allegra ® (allergies)Lupron ® (prostate cancer)Zoladex ® (prostate cancer)Diprivan ® (anesthesia)BuSpar ® (anxiety)Ventolin ® (bronchospasm)Hytrin ® (hypertension)Wellbutrin ® (depression)Zyrtec ® (rhinitis)Plavix ® (MI/stroke)Toprol-XL ® (hypertension)Tenormin ® (angina)Xalatan ® (glaucoma)Singulair ® (asthma)Diovan ® (hypertension)Harnal ® (prostatic hyperplasia)(Med Ad News 1999 Data).
GPCRs share a common structural motif, having seven sequences of between 22 to 24 hydrophobic amino acids that form seven alpha helices, each of which spans the membrane (each span is identified by number, i.e., transmembrane-1 (TM-1), transmembrane-2 (TM-2), etc.). The transmembrane helices are joined by strands of amino acids between transmembrane-2 and transmembrane-3, transmembrane-4 and transmembrane-5, and transmembrane-6 and transmembrane-7 on the exterior, or “extracellular” side, of the cell membrane (these are referred to as “extracellular” regions 1, 2 and 3 (EC-1, EC-2 and EC-3), respectively). The transmembrane helices are also joined by strands of amino acids between transmembrane-1 and transmembrane-2, transmembrane-3 and transmembrane-4, and transmembrane-5 and transmembrane-6 on the interior, or “intracellular” side, of the cell membrane (these are referred to as “intracellular” regions 1, 2 and 3 (IC-1, IC-2 and IC-3), respectively). The “carboxy” (“C”) terminus of the receptor lies in the intracellular space within the cell, and the “amino” (“N”) terminus of the receptor lies in the extracellular space outside of the cell.
Generally, when a ligand binds with the receptor (often referred to as “activation” of the receptor), there is a change in the conformation of the receptor that facilitates coupling between the intracellular region and an intracellular “G-protein.” It has been reported that GPCRs are “promiscuous” with respect to G proteins, i.e., that a GPCR can interact with more than one G protein. See, Kenakin, T., 43 Life Sciences 1095 (1988). Although other G proteins exist, currently, Gq, Gs, Gi, Gz and Go are G proteins that have been identified. Ligand-activated GPCR coupling with the G-protein initiates a signaling cascade process (referred to as “signal transduction”). Under normal conditions, signal transduction ultimately results in cellular activation or cellular inhibition. Although not wishing to be bound to theory, it is thought that the IC-3 loop as well as the carboxy terminus of the receptor interact with the G protein.
Gi-coupled GPCRs lower intracellular cAMP levels. The Melanophore technology (see infra) is useful for identifying Gi-coupled GPCRs.
Under physiological conditions, GPCRs exist in the cell membrane in equilibrium between two different conformations: an “inactive” state and an “active” state. A receptor in an inactive state is unable to link to the intracellular signaling transduction pathway to initiate signal transduction leading to a biological response. Changing the receptor conformation to the active state allows linkage to the transduction pathway (via the G-protein) and produces a biological response.
A receptor may be stabilized in an active state by a ligand or a compound such as a drug. Recent discoveries, including but not exclusively limited to modifications to the amino acid sequence of the receptor, provide means other than ligands or drugs to promote and stabilize the receptor in the active state conformation. These means effectively stabilize the receptor in an active state by simulating the effect of a ligand binding to the receptor. Stabilization by such ligand-independent means is termed “constitutive receptor activation.“